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"You've been delivering results that have made people's lives better," Perez said in his speech. "That's why Andrew Cuomo and (New York Lt. Gov.) Kathy Hochul are charter members of the accomplishments wing of the Democratic Party, and that's why I'm proud to endorse them."

Perez's nod came alongside endorsements for Cuomo from 2016 Democratic nominee Hillary Clinton and former Vice President Joe Biden. The one-two-three punch of prominent establishment leaders arriving at the convention to publicly back Cuomo angered Nixon supporters and dredged up grievances tied to the 2016 presidential primary, when the DNC's national apparatus favored, though never openly endorsed, Clinton over Vermont Sen. Bernie Sanders.

"Perez coming in at this time only further drives a wedge in the Democratic Party," said Jonathan Westin, the executive director of New York Communities for Change, a progressive activist group supporting Nixon. "What we're seeing play out, in a microcosm here in New York, is that the party elites are out of touch with where the base of the party is at."

First elected in 2010, Cuomo is facing a challenge from Nixon, who has attacked the governor for not pursuing a more aggressively liberal agenda. She trails Cuomo, who was backed by 95% of state party delegates on Wednesday, in the most recent polls by more than 20 points despite gaining ground since entering the contest a little more than two months ago.

"We're grateful for Chairman Perez's support and look forward to working with him to elect more Democrats at every level in November," Cuomo spokeswoman Lis Smith said on Thursday afternoon.

Perez was voted DNC chair in February 2017 over, among others, Minnesota Rep. Keith Ellison, who ran with the support of the Sanders coalition and has been vocal in advising the party to steer clear of the primary process. A DNC official cast Perez's decision to intervene here as more of a personal one, noting that the Buffalo-native and Cuomo have a personal relationship going back decades.

Ellison said in a statement Thursday, "The Democratic Party should not intervene in the primary process. It is our role to be fair to all contestants and let the voters decide."

The DNC has worked to mend post-2016 fences by using a "Unity Reform Commission" to tackle a suite of contentious intra-party disputes. Perez and Sanders also took part in a "unity tour" last spring in an effort to help soothe relations.

But New York City Councilman Jumaane Williams, who is running to unseat Hochul in the September primary, argued that the timing of the endorsement undermined that message.

"It seems like each time the DNC gets involved in a primary they support the establishment Democrats instead of the party's left leaning progressives," he said. "It would be nice if, at some point, calls for unity also mean supporting the left wing of the party."

"Cuomo can cloak himself in all the endorsements he wants, but it won't hide the fact that he's effectively governed as a Republican for eight years," she said. "Because he helped the Republicans maintain control of the Senate, we lost the chance to pass the DREAM Act and to protect women's reproductive rights, among other progressive policies."

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